It depends entirely on the situation really. Healer LBs are used only when things have really gone down the pan and everyone's almost dead/already dead - 99% of the time a Limit Break will be used by a DPS class to finish off a boss. I haven't yet encountered a situation where the tank LB is necessary but I'm sure there is one xD

Given you play healer, don't worry about it. You probably won't need to even touch your LB until you get to level 50. I've only ever used the healer limit break once, and that was in a Hard Mode Primal. In the dungeons you'll be running as you level, leave it for a DPS to do to get a chunk of the boss' HP down =3